Summary: Characterizing fish communities must be a priority to safeguard resources and determine critical changes. Here, species richness and the spatial and temporal evolution in the structure of fish assemblages were analysed based on photos taken in underwater free-diving contests. A total of 29 contests held from 2008 to 2015 at four different locations along the northeastern Spanish coast, including a marine protected area were analysed. Contests reward the number of species per participant and photographic quality. Species image frequency from each tournament were standardized to catch image rate. A total of 88 taxa were recorded, including 32 cryptobenthic species, the highest number recorded in the Mediterranean littoral system so far. Cluster analyses yielded four major groups. Catch image rates in the marine protected area were significantly higher for seven species of high commercial interest and for two big labrids of recreational interest, including an endangered species (Labrus viridis). Overall, the study showed that photographic free-diving contest data are a potential tool for determining species richness in littoral systems since contest rules promote competition between participants to obtain maximum fish diversity. We believe that this type of cost-effective data can be applied worldwide as a complementary way of monitoring littoral fish assemblage.

Another potential source of information is underwater photo contests. The technological developments in underwater photography have led to the expansion of a new sport activity: free-diving underwater photography. A recent event in underwater fish photography contests rewards the number of species photographed. Participants are given scores according to the number of species photographed, and they therefore search in every possible habitat to obtain the largest possible number of species. Consequently, the species richness resulting from these tournaments is expected to be higher than when traditional visual survey techniques are used. In Spain, and particularly in Catalonia, these tournaments began in the 1980s and have now become regular. The objective of this study was to assess species richness and classify fish communities based on the underwater free-diving fish photography contests held in Catalonia at four different locations from 2008 to 2015, including a no-take marine protected area (Medes Islands marine reserve).

This study analysed the information from the free-diving photography contests held in Catalonia, which was provided by the Catalan Underwater Activities Federation (FECDAS). The objective of these contests is to photograph only of live fish in the natural system while free-diving. The participants aim to photograph the highest number of fish species possible. Each participant can only present one photo per species so the number of photos per participant is equivalent to the number of species per participant. In Catalonia, the first official championship was held in 2005, and since then FECDAS has gathered the data from each contest, which includes the following information: date, location, number of participants, number of species and total number of photographs per species. In this study we only considered the championships held after 2007 when analogue cameras were completely replaced by digital ones.

A total of 29 tournament reports were analysed. Each tournament had on average 18 participants and lasted 5 hours. FECDAS provided us with a copy of the photographic database, which allowed us to validate the records of dubious species appearing in the contest reports.

Site features

The contests were held at four locations along the Catalan coast in northeast Spain (Fig. 1). Annual contests are held at the same locations and provide informative data for studying the spatial and annual variability of littoral fish communities. In addition to their latitudinal differences, the four locations also have different habitat features, and these differences should also be reflected in their fish assemblages.

The Medes Islands, located in the northern area, are a group of islands ~1 km from the mainland coast. This archipelago is one of the oldest marine reserves in the western Mediterranean. It has been protected since 1983 and has become a popular scuba diving destination. The contests were located on the southern side of the main island, at depths ranging from 0 to 20 m and characterized by diverse habitats: rocks, sand, Posidonia oceanica meadow and coralligenous. The second location, also in the northern area and near to the Medes Marine Reserve, was a coastal zone of Palamós, Margarida Cove, which has a diverse habitat: rocks, breakwater blocks, sand and P. oceanica meadow, at depths ranging from 0 to 15 m. The third location was Mataró, where the contest site was located ~500 m offshore from the coast at depths ranging from 6 to 12 m. The location is restricted to a natural rock barrier that is over 150 m long, has many cracks and cavities, and is surrounded by sandy beds with some P. oceanica patches at the eastern extreme. Finally, the fourth location was L’Ametlla, a coastal area at Port d’Estany, which has brackish water of depths between 0 and 10 m with a similar bottom structure: rocks, sand and a large P. oceanica meadow.

Data analysis

Species relative abundance data from the tournament information were estimated from the catch image rate for each location, which was estimated as the total number of reported photos per species divided by the total number of participants. Taxonomic resolution was not fully attained and some species were classified at the level of genera (Atherina and Sphyraena) or family (Mugilidae).

Absolute number of species richness (S) and Margalef index, α=(S–1)/ln (N) (N total number of images) were estimated as measures of species richness and diversity for each contest. One-way ANOVAs using STATISTICA 10 (Stat-Soft Inc. 2010) were performed to assess potential differences in species richness and diversity (α) among locations. As the number of samples varies among localities the weighted mean was used in the analysis. The spatial structure and fish community was analysed using the PRIMER software package (Clarke KR and tutorial. PRIMER-E) and agglomerative hierarchical clustering was applied. A community similarity matrix based on the Pearson correlation coefficient was calculated for hierarchical group average linking. Catch rates were not transformed because participants could only present a single photo per species, regardless of the number of photos or encounters they had with the most frequent species. Consequently, the most frequent species were already down-weighted by the contest rules, but the information of species relative frequency was retained because it is equivalent to the probability of being photographed.

In total, 16307 photographs corresponding to 29 different contests were analysed. The mean number of participants per contest was 17 and the average number of photographs per participant was 33. The number of photographs per participant has increased over recent years at two locations, Palamós and the Medes Islands, with no effect on the total number of species recorded (Table 1).

Table 2. – Fish species average catch image rate and standard deviation by location. The letters within brackets next to the species correspond to those with a catch image rate significantly higher in the Medes Marine Reserve; each letter corresponds to the location where the significant differences were observed. Asterisks indicate a species described for the first time in the Reserve.

Cluster analysis based on the species catch rates by location and year yielded four distinctive clusters (Fig. 3) corresponding to each specific location. Regardless of the year, each location was grouped in the same cluster, with one exception, the Medes Islands in 2015. The silhouette analysis (mean silhouette width=0.46) revealed that all contests had positive silhouette widths, which is indicative of correct classification within groups. The two-dimensional ordination of the 29 contests (Fig. 4) yielded a moderate level of ordination (stress=0.14), reflecting cluster separation through a gradual continuum of change. A gradient from north to south was observed among locations near the shoreline (Medes, Palamós and L’Ametlla). Consistently, the results of the cluster analysis showed that the most dissimilar group was represented by the contests held in Mataró.

The first cluster, grouping all the samples from Mataró, had a low number of gobiid and bleniid species, scarcity of mugilids and the absence of Atherinidae and European seabass (Dicentrarchus labrax), which mainly marked the difference of this group. An additional difference was the high values of species such as Scorpaena notata, Thalassoma pavo, Boops boops and Gobius geniporus.

The second cluster grouped all the contests of L’Ametlla de Mar, the southernmost location (the last contest held at the Medes Islands was also part of this group). The main differences compared with other groups were the high abundance of the genera Pagellus, Symphodus cinereus and Parablennius incognitus together with the scarcity of Ctenolabrus rupestris, Microlipophrys canevae, Symphodus melanocercus and Diplodus cervinus. In this group the relative abundance of European seabass was also high, and only comparable with the values observed at the Medes Islands.

The third and fourth cluster comprised the contests held at the northernmost location corresponding to a non-protected and protected area respectively with the exception of the most recent year (see above) for the latter group. The main features of this group were the low relative abundance of gobiids, the maximum observed abundance of some species of labrids (Symphodus melanocercus, Symphodus viridis and Labrus merula) and the scarcity of S. ocellatus. This group was also characterized by high abundance of commercial species (e.g. Epinephelus marginatus, Conger conger and Phycis phycis), including the red scorpion fish (S. scrofa), but it also had the lowest abundance of the other three species of the same genus (S. notata, S. porcus and S. maderensis).

Although the clusters did not show any temporal variability within locations, except for the last contest held in Medes Islands, some shifts can be depicted. Some temporal changes were observed in the contest distances within the Medes Islands cluster, in which the 2013 and 2014 contests were clearly separated from the earlier ones. Similarly, differences were observed within the Palamós (cluster C) contests: those from 2012 onwards were distant from the previous ones, while in L’Ametlla the temporal pattern was more gradual.

The post hoc test showed significantly higher catch rates at the Medes Islands MPA with respect to the other locations (Table 2). These higher rates were mostly observed in species of commercial interest such as P. phycis and E. marginatus, but the list can be extended to Conger conger, D. labrax, S. umbra, S. scrofa, D. cervinus and D. sargus. In addition, the MPA also showed significantly higher catch rates for two species of labrids that are of negligible commercial interest but are highly targeted by spear fishers (L. merula and L. viridis). Furthermore, the relative abundance of S. cinereus decreased from southern to northern locations, while S. melanocercus displayed the opposite pattern.

The type of information used in this study has several weaknesses, such as a potential underestimation of the most frequent species because the number of photographs per species and participant is limited to one, and the possibility that the same fish could be photographed by different participants. However, despite the potential weaknesses of photographic free-diving contest data, the results of this study show their effectiveness for evaluating species richness, in particular of cryptobenthic species, and for analysing littoral fish communities. Furthermore, it has also proved its worth as a tool for updating fish inventories. We can summarize that this type of contest enhances competition between participants to obtain maximum fish diversity. Additional positive aspects of this information source are that it is cost-effective, non-destructive, a potential observatory, an platform for interaction between scientists and free divers, and an alternative for spear fishers in MPAs. We conclude that the monitoring of photographic free-diving contests could be a complementary information source to scientific monitoring.

The authors are very grateful to the Catalan Federation of Underwater Activities (FECDAS) for providing all the information analysed in this study and in particular to Carles Font, delegate of apnoea photo-hunting and responsible for compiling the database of these contests and to Catherine Stonehouse for revising and improving the original English manuscript. We also thank the anonymous referees for their useful comments.

Kovačić M., Sanda R. 2016. A new species of Gobius (Perciformes: Gobiidae) from the Mediterranean Sea and the redescription of Gobius bucchichi. J. Fish Biol. 88: 1104-1124.https://doi.org/10.1111/jfb.12883